Inhibitors of angiogenin

ABSTRACT

An inhibitor of angiogenin and method of use of the inhibitor, wherein the inhibitor inhibits the angiogenic activity of angiogenin. The inhibitor is dispensed at a suitable concentration within a physiologically compatable medium, suitable for administration to an animal in sufficient quantity to inhibit the biological activity of naturally occurring angiogenin within the animal.

BACKGROUND OF THE INVENTION

Angiogenin is a human protein which induces blood vessel formation (Fett et al. 24 Biochemistry 5480, 1985). This biological activity is expressed with an amount as low as 35 fmol (using a chick embrYo CAM assay procedure, Id.). Although originally isolated from medium conditioned by human tumor cells,aangiogenin is not tumor specific, can be found in a variety of other cells and biological fluids, and most likely plays a role in normal and/or pathological neovascularization. It has a molecular weight of about 14,400 and an isoelectric point greater than about pH9.5, Id. Strydom et al. 24 Biochemistry 5486, 1985 also disclose the amino acid sequence of angiogenin.

Angiogenin is known to have an enzymatic activity. Specifically, it catalyzes limited cleavage of 28S and 18S rRNA to produce a specific pattern of products of 100-500 nucleotides in length (Shapiro et al., 25 Biochemistry 3727, 1986), but it has no significant ribonuclease activity in standard RNase assays, Id.

SUMMARY OF THE INVENTION

We have discovered that substances inhibiting at least the angiogenic activity of angiogenin can be used in methods and compositions for inhibiting tumor growth. Moreover, substances inhibiting the above described 18S, 28S rRNA-degrading enzymatic activity of angiogenin are effective tumor suppressants.

Accordingly, the invention features a method of inhibiting growth of a tumor in an animal, comprising administering an inhibitor of the angiogenic activity of angiogenin in sufficient quantity to inhibit the angiogenin activity in the tumor.

In a second aspect, the invention features a method of inhibiting growth of a tumor in an animal, comprising administering to the animal an inhibitor of the following specific enzymatic activity: cleavage of S or 28S rRNA to generally yield segments of 100 to 500 bases.

The following are features of preferred embodiments of the above methods: The inhibitor is able to bind to RNaseA or to angiogenin; the inhibitor is the complete natural molecule or segment or derivatives thereof having the ability to inhibit the above described enzymatic activity, and most preferably comprises a segment of a specific substance known as human placental RNase inhibitor (PRI) having the ability to inhibit the above described enzymatic activity of angiogenin; other proteins analogous to PRI from other human tissue or from other mammals also can be used; the inhibitor is administered at 10-10,000 μg/kg body weight of the animal; and the animal is a human.

In a third aspect, the invention features an inhibitor capable of inhibiting the angiogenic activity of angiogenin, wherein the inhibitor is dispersed within a medium that is physiologically compatible -i.e., suitable for administration to an animal-at a suitable concentration and in sufficient quantity to inhibit the angiogenic activity of naturally occurring angiogenin within at least a defined area of the animal, for example, the area immediately surrounding a tumor.

In preferred embodiments of this aspect, the inhibitor inhibits the enzymatic activity of angiogenin; and most preferably the inhibitor comprises a segment of PRI (or an analogous protein from other human tissue or from mammalian tissue) having the ability to inhibit the 18S, 28S rRNA-degrading activity described above.

In a fourth aspect, the invention features a method of inhibiting disorders associated with neovascularization. The method comprises administering to an animal an inhibitor of the angiogenic activity of angiogenin in sufficient quantity to inhibit angiogenic activity associated with the disorder.

In a fifth aspect, the invention features a therapeutic composition comprising a polypeptide comprising the amino acid sequence Val-Asn-Pro-Ala-Leu-Ala-Glu-Leu-(Asn-Leu-Arg), Ser-Asn-Glu-Leu-Gly-Asp-Val-Gly, or Trp-Leu-Trp-Leu-Ala-Asp-(Gln-Asp-Lys).

In preferred embodiments, a segment of the polypeptide has angiogenin inhibiting activity.

In a sixth aspect, the invention features engineered nucleic acid encoding the above amino acid sequences. (Engineered nucleic acid is defined below, briefly it refers to any nucleic acid removed from its natural environment.) Preferably, the nucleic acid is DNA and has the sequence 5'-TGGBTNTGGBTNGCNGABCADGABBAA-3', 5'-AABGADBTNGGNAGABGTNGG-3', or 5'-GTNAABCCNGCNBTNGCNGA-3' (where B is C or T, D is A or G, and N is A, T, C, or G). In a seventh aspect, the invention features engineered nucleic acid encoding a segment of a polypeptide having angiogenin inhibitory activity. Preferably, the nucleic acid is obtained by screening a mammalian gene library with a probe corresponding to a segment of a polypeptide having angiogenin inhibitory activity; the gene library comprises genomic or cDNA; inhibitor; the library comprises human DNA, or human cDNA; and the segment is Val-Asn-Pro-Ala-Leu-Ala-Glu-Leu-(Asn-Leu-Arg), Ser-Asn-Glu-Leu-Gly-Asp-Val-Gly, or Trp-Leu-Trp-Leu-Ala-Asp-(Gln-Asp-Lys).

In an eight aspect, the invention features a method for making a polypeptide having angiogenin inhibitory activity. The method comprises expressing engineered nucleic acid encoding the polypeptide in a host cell. Preferably, the polypeptide is a segment of human placental ribonuclease inhibitor.

This invention provides a means for preventing or reducing growth of tumors. The preferred inhibitor, over angiogenin and other PRI-binding proteins in the body fluids, or within a specified local area, such as that area immediately surrounding a tumor, and thus need only be provided at very low concentrations.

Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof and from the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Figures will first briefly be described:

DRAWINGS

FIG. 1 is a graphical representation of the effect of angiogenin on inhibition of RNaseA activity toward uridylyl (3', 5.) adenosine (UpA) by PRI (Blackburn, 254 J. Biol. Chem. 13484, 1979) as an example of an angiogenin inhibitor.

FIG. 2 is a graphical representation of elution of angiogenin (A) or angiogenin and PRI (B) from an HPLC column.

STRUCTURE

Anqiogenin

Angiogenin is a protein, having activities as described above, which can be obtained and purified as described by Fett et al., supra, and Shapiro et al., supra. Alternatively, angiogenin can be obtained and purified using recombinant DNA techniques. Kurachi et al., 24 Biochemistry 5494, 1985, describe the cDNA and gene encoding for angiogenin; this cDNA or gene can be expressed from a standard expression vector and the resulting angiogenin protein purified by standard Procedure, e.g., in mammalian, yeast or various microbial expression systems. Id. at 5498.

Angiogenin Inhibitors

An angiogenin inhibitor is any compound able to inhibit the angiogenic activity of angiogenin. Preferably the inhibitor of angiogenin is a protein; most preferably it is a protein capable of binding to angiogenin and of inhibiting at least its biological angiogenic activity and preferably also its enzymatic 28S/18S rRNA-degrading activity. (The enzymatic activity is measured as described by Shapiro et al., 20pra. Briefly, 15-20μg RNA is incubated with about 1.9μM angiogenin at 37° C. in either 30 mM Hepes or 20 mm Tris, pH 7.5, containing 30 mM NaCl in a total volume of 13.5 μ.The reaction is terminated after about 90 minutes using 48μl of a formamide/formaldehyde reagent, Id.) Preferably, the inhibitor is isolated from mammalian tissues, most preferably from human placental tissue. One example of the inhibitor, PRI, can be isolated as described by Blackburn (254 J. Biol. Chem. 12,484, 1979). Proteins which are substantially equivalent to PRI, that is they have similar amino acid compositions, and have similar biological and enzymatic inhibitory activities, can be found not only in other human tissues but in other mammalian tissues. For example, the various inhibitors of RNAse A described by Burton et al., (19 Int. J. Peptide Protein Res. 372, 1982, herby incorporated by reference) may be suitable in this invention. These proteins have about 70-80% similarity in amino acid composition to PRI, as shown in Table 1 (taken from Burton et al., Id.). Such proteins are also angiogenin inhibitors for purposes of the invention. Methods for isolating and purifying these proteins are given below, using standard affinity chromatography methodology, or recombinant DNA techniques.

                  TABLE 1                                                          ______________________________________                                         Amino acid compositions of various liver RNase inhibitors and                  human placental RNase inhibitor                                                        Residues/molecule of inhibitor                                                 Liver              Placenta                                            Amino acid                                                                               Beef   Mouse   Pig  Rat  Sheep Human                                 ______________________________________                                         Asx       47     55      47   59   48    47                                    Threonine 21     22      25   23   19    16                                    Serine    45     43      43   43   42    45                                    Glx       62     63      64   60   62    64                                    Proline   15     16      15   17   17    17                                    Glycine   47     33      43   33   51    36                                    Alanine   37     30      35   29   35    34                                    Valine    24     20      19   22   23    24                                    Methionine                                                                                2      3       2    2   1-2   2-3                                   Isoicucine                                                                                9     13       9   10   10    12                                    Leucine   91     94      93   89   92    85                                    Tyrosine   5      5       4    5    4     4                                    Phenylalanine                                                                             4      3       2    5    4     6                                    Histidine  6      3       9    6    5     6                                    Lysine    14     21      15   20   15    17                                    Arinine   19     19      21   20   20    23                                    1/2 Cystine +                                                                            30     26-27   35   31   27    30                                    cysteine                                                                       Tryptophan                                                                                5      5       5    5    6     5                                    Total residues                                                                           483    475     486  479  482   473                                   ______________________________________                                    

Alternatively, the gene encoding an inhibitor may be cloned by standard techniques. Such techniques include purifying the inhibitor, determining part of its amino acid sequence, creating a DNA probe capable of coding for this amino acid sequence, and using the probe to detect recombinant vectors in a cDNA or genomic library created from, e.g., placental cells. The cloned gene can then be expressed in any suitable expression vector in a suitable expression host cell, e.g., bacteria, yeast, or tissue culture cells. The recombinant inhibitor protein produced by those cells can then be purified from the culture supernatant or from the cells.

We have determined partial amino acid sequences of three tryptic segments of PRI, using standard procedures. The segments were purified by HPLC and analysed for amino acid sequence. These sequences are

Val-Asn-Pro-Ala-Leu-Ala-Glu-Leu-(Asn-Leu-Arg); Ser-Asn-Glu-Leu-Gly-Asp-Val-Gly, and Trp-Leu-Trp-Leu-Ala-Asp-(Aln-Asp-Lys).

Amino acids are represented by their standard three letter codes; those in brackets have not been conclusively determined. These sequences can be used to generate oligonucleotide probes suitable for detecting clones having DNA encoding PRI, or for related proteins. For example, suitable oligonucleotides can be designed as described by Lathe 183 J. Mol. Biol., 1985. For example, the above amino acid sequences are encoded by the sense strand 5'-TGGBTNTGGBTNGCNGABCADGABAA-3 Where B is C or T, D is A or G and N is A, T, C, or G. A suitable probe for the corresponding anti-sense strand is ##STR1## where B is C or T, I is Inosine (which can base pair with A, T, or C) ##STR2## is C or I.

Inosine is used in the probe sequence is place of A, T, and C so that the number of probes to be synthesized as a mixture can be minimized without prejudicing their hybridizing ability to the natural gene or cDNA sequences. This anti-sense probe is particularly suitable for probing a human placental cDNA library, using stringent hybridization conditions. An alternative probe is one in which preferred codons are used to design the probe. Although the sequence is shorter than preferred, one example of the above sense strand using preferred codons is: 5'-TGGCTGTGGCTGGCCGACCAGGAGAA-3'The antisense (or homologous) strand of this probe is also suitable. Equivalent such oligonucleotide probes for the other two amino acid sequences given above are: ##STR3##

Those skilled in the art will realize that these examples can be readily supplemented from the information presented above, and by analysis of other peptide fragments.

One example of a method to isolate a cDNA clone of the gene encoding PRI is to use the above three antisense strand oligonucleotide probes under stringent hybridization conditions to probe a cDNA library of human placental DNA. Any clones which hybridize to one or more of the probes, and preferably hybridize to all three, are potention clones of the PRI-encoding gene. To determine whether the clones do encode PRI they can be purified, and hybridization with the probes rechecked. Ultimately, the clones can be sequenced and the sequences compared to see if they correlate with the known properties of PRI, i.e., the amino acid sequence, molecular weight and amino acid composition. Expression of PRI can then be achieved by insertion of the cDNA clone into an expression vector and the recombinant PRI which is expressed, isolated, and purified. All such clones contain engineered DNA, that is, DNA taken from its natural environment and inserted into a vector, such as a plasmid or phage, or even within the genome of an organism. Thus, the engineered DNA is no longer surrounded by naturally occuring sequences on either side of it. Generally such engineered DNA is constructed using recombinant DNA techniques, and does not include naturally occurring DNA is which, for example, a translocation of chromosomal DNA in vivo has changed the environment of DNA; however it does include such natural events which occur after the DNA has been manipulated in some way by recombinant DNA methodology.

Once cloned, the naturally occurring gene may also be modified by standard techniques, for example, by altering the amino acid sequence of the inhibitor, so long as the inhibitor retains the ability to inhibit the biological angiogenic activity of angiogenin. Such alterations may be designed to increase the binding ability of the inhibitor for angiogenin. Further, related inhibitor-encoding genes can be isolated by using a part of the above described cloned gene encoding an inhibitor as a probe for libraries, constructed by standard techniques, of other animal genomes.

In order to isolate other inhibitors, one possible procedure includes detecting proteins able to bind angiogenin, for example, by binding angiogenin to a column and passing a sample containing potential inhibitor proteins through this column. (RNaseA can be used in place of angiogenin, as described by Blackburn 254 J. Biol. Chem. 12488, 1979.) Bound protein can then be eluted, for example, by using 0.lM sodium acetate pH 5.0 containing 3M NaCl, 15% glycerol, lmM EDTA and 5mM DTT, which is able to dissociate the angiogenin/inhibitor complex. The eluted protein can then be purified and tested, as described below, to see if the released proteins inhibit the angiogenic activity of angiogenin, or alteratively if they inhibit its enzymatic activity. The active protein(s) may then be purified by standard procedure.

Further, it is possible to use segments of inhibitors, such as segments of PRI, which retain the biological inhibiting character of the native inhibitor. Such segments can be created by standard procedure using proteases to digest natural inhibitors to produce active segments; or by using recombinant DNA techniques to remove non-essential parts of the structural gene or cDNA encoding the inhibitor, and then expressing this engineered DNA to produce a modified inhibitor.

Inhibitory Activity

Below is an example of in vitro measurements relative to the inhibitory effect of PRI on angiogenin. This example is not meant to be limiting for the invention and those skilled in the art will realize that these examples are indicative of whether other inhibitors or segments thereof can be used in the methods described below.

Measurement of Inhibition of the Enzymatic Activity of Angiogenin

Angiogenin and PRI were purified generally as described by Fett et al. and Shapiro et al., cited above. The enzymatic activity of angiogenin was measured as described by Shapiro et al., supra. Briefly, angiogenin catalyzes the cleavage of 28S and 18S rRNA to form products of 100 to 500 nucleotides in length. These products are stable and include a large, approximately 500 bases, segment which is visible in an agarose gel. In one experiment, 12 .sub.μ g of RNA was incubated with or without angiogenin and/or PRI at 37° C in 33 mM Hepes, and 33 mM NaCl, pH7.5. After 30 min the reaction was terminated, as described by Shapiro et al., supra, the samples run on a 1.1% agarose gel, and stained with ethidium bromide. 0.96 .sub.μ M PRI completely inhibited the enzymatic degradation of rRNA by 0.8 .sub.μ M angiogenin. PRI alone had no activity on rRNA. Thus, PRI is a potent inhibitor of the enzymatic activity of angiogenin, being active at only just over a 1:1 ratio with angiogenin.

Stability of the Anqiogenin/PRI Complex

For therapeutic use of the angiogenin inhibitor it is advantageous to use an inhibitor that binds strongly to angiogenin and reduces its biological activity. PRI is such an inhibitor.

RNaseA also binds to PRI and competes with angiogenin for binding PRI in solution. The enzymatic activity of RNaseA is also inhibited by PRI. These properties can be used to determine the stability of the PRI/angiogenin complex, as follows.

UpA is an excellent substrate for RNaseA but not for angiogenin. In one experiment, a varying amount of angiogenin was added to a mixture of 0.27 nM RNaseA and 0.20 nM PRI, with the RNaseA and angiogenin being mixed together first, followed by PRI for 5 min. at 25° C., and then the UpA substrate (0.2mM, in a buffer of 10.sub.μ g/ml human serum albumin (HSA), 0.1 M 2-(N-morpholino)-ethane-sulfonic acid, 0.1M NaCl, and mixture the more PRI that will be bound to it, and thus the less PRI that will be bound to RNaseA and able to inhibit its enzymatic activity. Further, the stronger the binding of PRI to angiogenin than to RNaseA, the greater the reduction in inhibition of RNaseA activity, since PRI will then preferentially bind to angiogenin. The results are shown in FIG. 1. Relative inhibition is calculated as (Vo-Va)/(Vo-Vr) where Vo denotes velocity of RNaseA activity in the absence of PRI, Vr denotes velocity in the presence of PRI with no angiogenin added, and Va is velocity in the presence of PRI with angiogenin added. At 5.8 nM angiogenin there is essentially no inhibition of RNaseA activity. Thus, a large excess of angiogenin is needed to remove all the PRI from reacting with and inhibiting the RNaseA.

In a related experiment, the K_(i) for angiogenin and PRI can be estimated using the above method and modifying it so that the PRI and angiogenin are firstly preincubated for 10 minutes, followed by addition of RNaseA and UpA. This assay determines the level of free inhibitor remaining after incubation of PRI and angiogenin. Thus, if the dissociation of PRI and angiogenin is slow there will be little inhibition of RNaseA activity. At a rato of angiogenin:PRI of 1:1.2 no free PRI was detected. Thus, PRI and angiogenin are tightly bound and appear to have a K_(i) less than 0.1 nM.

The strength of binding of PRI and angiogenin was also demonstrated by cation-exchange HPLC. This process can distinguish free angiogenin from angiogenin/PRI complex. A Synchropak CM 300 column (250×4.1 mm; Sychrom, Inc.), a Waters Associates liquid chromatography system and a Hewlett-Packard 3390A integrator were used; the results are shown in FIG. 2. Samples of lml in 0.lM Tris, pH7, containing lmM EDTA and 10μg HSA were eluted with a 10 min. linear gradient from 220-620 mM NaCl in 20 mM sodium phosphate, pH7, at a flow rate of lml/min.; effluents were monitored at 214 nm. In panel A is shown elution of 0.64 μg angiogenin; in panel B the elution of 0.64 μg angiogenin and 12 μg PRI. There is no detectable free angiogenin shown in panel B. Addition of RNaseA to the angiogenin-PRI mixture at 32-fold excess did not produce free angiogenin, even after 17 h incubation. Thus dissociation of angiogenin/PRI appears to have a half life of greater than 1 day.

Inhibition of Angiogenic Activity

Angiogenesis (i.e., angiogenic activity) was assessed by a modification of the chick embryo chorioallantoic membrane (CAM) assay of Knighton et al. 35 Brit. J. Cancer 347 (1977) described by Fett et al., suora. For this assay PRI was desalted using an Amicon Centricon-10 microconcentrator to dilute the buffer at least 500 fold. This removes interfering components, from the PRI solution, which may adversely affect the experiment or may harm the egg itself. The results of adding PRI to the angiogenin are shown in Table 2 below.

Data for experiments #1 and #2 represent composites of results obtained in 2 and 3 sets of assays, respectively. Between 10 and 20 eggs were utilized for each of the three experimental groups within each set. The following amounts of angiogenin and PRI were employed: Experiment #1, 75 ng and 2 μg, respectively; Experiment #2, 46 ng and 700 ng; and Experiment #3, 25 ng and 180 ng.

                  TABLE 2                                                          ______________________________________                                         % Positives (Total Number of Eggs)                                                         Experiment                                                         Sample        #1          #2      #3                                           ______________________________________                                         Angiogenin    58 (24)     52 (54) 62 (13)                                      PRI           17 (29)     33 (48) 17 (12)                                      Angiogenin + PRI                                                                             15 (26)     25 (52)  7 (14)                                      ______________________________________                                    

From these results it is evident that excess PRI decreases angiogenin activity to a level indistinguishable from that observed with the buffer and inhibitor alone controls.

Inhibition of Tumor Growth

For immunotherapeutic studies, male nude (nu/nu) mice (Charles River Laboratories) were maintained under laminar flow conditions, and were age- and cage-matched (5 animals per cage) prior to experimentation. Experimental animals (5-10 per group) were injected subcutaneously (S.C.) with 5×105 HT-29 human colon adenocarcinoma cells (Fogh et al., In Human Tumor Cells In Vitro, pp115-160 ed. Fogh, Plenum Press, New York., 1975) on day 0. Also on day 0, animals were treated with either buffer control (the buffer used to store PRI dialysed against phosphate buffered saline; 0.2g/l KCl, 0.2g/l KH₂ PO₄, 8g/1 NaCl and 2.16g/1 Na2HPO .7H20, pH7.4 at 37° C., PBSA 100 .sub.μ l) or placental inhibitor (100 .sub.μ l) at varying dosages by S.C. or intraperitoneal (I.P.) injection. Treatment regimens included daily injections (10-11 doses) or injections at 2-3 day intervals (5-10 doses) depending on the experimental protocol. Assessment of animal health, weight, tumor size as well as photographic records were recorded 2-3 times per week. At the termination of the experiments blood and tissue samples were collected for immunological and histological evaluation.

In one experiment, forty male nu/nu mice were injected S.C. with 5-105 HT-29 cells on day 0. Additionally, on day 0 mice 1-10 (Group 1) received 100 .sub.μ of control buffer; mice 11-20 (Group 2) received 10 .sub.μ g in 100 .sub.μ l of placental inhibitor; mice 21-30 (Group 3) received 1.0 .sub.μ g in 100 .sub.μ of placental inhibitor; and mice 31-40 received 0.1 μg in 100 .sub.μ l of placental inhibitor. Groups 1 and 2 received further injections of buffer or 10 μg of placental inhibitor, respectively, on days 1, 4, 6, 8, 18, 20, and 22. On days 1, 4, 6 and 8, Groups 3 and 4 received placental inhibitor at dosages of 1.0 μg or 0.1 μg, respectively. All treatment injections were given I.P. After 60 days, only animals 13, 14, 17, 18, 20 and 24 remained tumor free. Thus, inoculation with 10.sub.μ g PRI prevented tumor formation in about 60% of animals.

Inhibition of both the biological and enzymatic activities of angiogenin by an inhibitor has important mechanistic, physiologic and pharmacologic implications. It is consistent with the hypothesis that these two actions of angiogenin are interrelated, as suggested previously by the simultaneous loss of both activities upon carboxymethylation by bromoacetate at pH 5.5. Shapiro et al., supra. Further it raises the possibility that such inhibitors may play a role in the in vivo regulation of angiogenin. The angiogenin/PRI interaction likely involves regions of angiogenin separated widely in the three-dimensional structure, many of them outside the active center. Thus, conservation of residues necessary for enzymatic activity alone probably cannot account for the strength of the interaction.

This implies that the capacity of angiogenin to bind PRI has been maintained independently during evolution. Apparently, binding by PRI and other inhibitors reflects a physiologically relevant control mechanism having pharmacologic and therapeutic potential. For example, as shown above, these inhibitors are active in prevention of tumor formation in mice. Apparently they are suitable for injection into humans or other animals in order to prevent tumor growth, and other diseases associated with neovascularization, e.g., diabetic retinopathy, rheumatoid arthritis, and Kaposi's sarcoma. That is, they are suitable for treating disorders where vascularization plays an important role in the pathophysiology of diseases such as solid tumors, hemangiomata and psoriasis.

Preferably, they are injected intravascularly, most preferably intravenously, or injected intraperitoneally in a physiologically compatible medium (such as PBSA, or those buffers and agents described by Krakoft, Ca-A Cancer Journal for Clinicians, March/April 1987, Vol. 37:93), suitable for administration to an animal in a sufficient quantity, e.g., about 10-10,000 g/kg body weight of the animal, to inhibit naturally occuring biological activity of angiogenin within the animal. Alternatively, they can be administered topically to a specific area, or injected subcutaneously.

We have found that these inhibitors are stable for at least 2 weeks even at concentrations as low as 8μM when they are in a purified form; there is no need to include DTT in the storage buffer or the therapeutic composition being administered, however, natural agents, which are physiologically compatible, e.g., N-acetyl cysteine or cysteine or cysteamine may be used. For administration it is important that the buffer fluid not injure the patient. It is possible that these inhibitors may be used prophylatically at a low concentration. Thus, for example, it is appropriate to administer 10-10,000 μg/kg body weight to patients at risk of cancer development, e.g., patients who have had a primary tumor removed.

Other embodiments are within the following claims. 

What is claimed is:
 1. A cloned DNA molecule comprising a nucleic acid sequence encoding the amino acid sequence

    Val-Asn-Pro-Ala-Leu-Ala-Glu-Leu


2. A cloned DNA molecule comprising a nucleic acid sequence encoding the amino acid sequence

    Ser-Asn-Glu-Leu-Gly-Asp-Val-Gly


3. A cloned DNA molecule comprising a nucleic acid sequence encoding the amino acid sequence

    Trp-Leu-Trp-Leu-Ala-Asp 